7 × 7 is the coffee-break Shikaku: 49 cells, roughly nine to fifteen rectangles, and enough room for the first real deduction chains. Prime clues (7s especially) are your anchors here — a 7 must be a 1×7 strip, and on a seven-wide board there are only so many places it can lie.
This is also the size where the medium and hard tiers start to bite: you'll meet boards where no clue has a single obvious rectangle, and progress comes from spotting the cell that every candidate of one clue covers — which rules that cell out for everyone else.
Work through the medium tier here before moving up to 10 × 10 boards, or make it a habit with the daily puzzle.